1. Field of the Invention
The present invention relates to a three-dimensional medical imaging apparatus used, for example, in surgical procedures, particularly in neurosurgery, ear, nose and throat surgery, orthopedic and plastic surgery, obstetric surgery, or ophthalmic surgery.
This application is based on Japanese Patent Application No. 2004-350265, the content of which is incorporated herein by reference.
2. Description of Related Art
In the related art, surgical microscopes are used to carry out magnified examination of minute operative sites in neurosurgery and so on. With organs formed of minute tissue, such as the brain, it is difficult to identify their tissue structure with the naked eye, and therefore, the treatment of these organs is normally carried out under a microscope. In neurosurgical procedures, which are carried out in very small regions on extremely critical and delicate tissue like blood vessels and nerves, not only is the tissue observed, but treatment such as joining and rerouting blood vessels and nerves, removing tumors, and the like is actually performed. Accordingly, it has become an important function of conventional surgical microscopes to allow the users to three dimensionally recognize an object to be examined in order to perform the treatment as well as allowing observation of a magnified image of the object to be examined.
Generally, humans obtain various kinds of information when observing a three-dimensional object with the eyes. This information includes binocular parallax, perspective (that is, distant objects are diminished and near objects are magnified), blurring in the depth direction, occlusion of objects, experience, knowledge, memory, and so forth. Here, it is assumed that an unknown specimen (an object that has never been seen before) is placed on a level platform and examined by illuminating it from all sides so that there are no shadows (conditions which are common in microscopy). In such a case, of the information for three-dimensionally capturing the object described above, occlusion, knowledge, experience, and so forth are not useful, but binocular parallax is effective in helping to determine the depth of the object. Surgical microscopes provide a three-dimensional view using binocular parallax to obtain depth information about the object under examination (for example, see Japanese Unexamined Patent Application Publication No. HEI-10-282428).
The surgical microscope disclosed in the above-cited Japanese Unexamined Patent Application Publication No. HEI-10-282428 includes two beams, one for the right eye and one for the left eye, and is configured such that the two beams pass through an eyepiece optical system and are observed. The operator sees the right and left images of the object under examination, which have parallax, with his or her right and left eyes, respectively, and these images are fused in the operator's brain, thus allowing a three-dimensional image of the object under examination to be observed.
Recently, instead of eyepiece optical systems, three-dimensional medical imaging apparatuses including imaging devices located near the image-forming planes in the left and right light paths have begun to be used. Three-dimensional medical observation apparatuses have also begun to be used which are configured such that three-dimensional observation of the object under examination is achieved by displaying images acquired by the three-dimensional medical imaging apparatus on a three-dimensional display device.
The field angles of a surgical microscope in the second embodiment disclosed in Japanese Unexamined Patent Application Publication No. HEI-10-282428 for various working distances (WD) and fields of view are shown in Table 1.
TABLE 1Objective focalField of viewField angleField anglelength (mm)(mm)(tan θ)(degree)33529.10.0251.46138535.20.0392.23246544.90.0543.10733514.50.0020.12438517.60.0110.61446522.40.0201.1613357.20.0311.7493858.80.0301.70546511.20.0291.676
As shown in Table 1, all field angle are extremely small (a few degrees). Surgical procedures using surgical microscopes generally involve treatment, not just examination, and therefore a surgical instrument or the like for performing treatment on the operative site is frequently moved in the observation field of view.
In such a case, if the field angle is narrow, there is no substantial change in the size of the observed image with depth, even if the surgical instrument is moved in the optical axis direction of the objective optical system. In other words, there is no perspective. Normally, when a person carries out observation with the naked eye, near objects are magnified and distance objects are diminished. Therefore, a sense of depth is obtained based on the degree of change in size of the object observed with the naked eye as the object moves.
This point is described in more detail in FIGS. 12A to 12C and FIGS. 13A to 13C.
FIGS. 12A to 12C illustrate the sense of depth when observing an object that moves in the direction of the line of vision of an observer when the field angle is small. FIGS. 13A to 13C show the sense of depth when observing an object that moves in the direction of the line of vision of an observer when the field angle is large. FIGS. 12A and 13A show the object approaching the observer, and FIGS. 12B, 12C, 13B, and 13C show the observation fields of view when the object approaches the observer.
Even though a sphere becomes closer to and further away from the observer in the same way, the view perceived by the observer when viewing the sphere differs depending on the different field angles.
More specifically, as shown in FIGS. 12A to 12C, when the field angle is narrow, it is difficult to obtain any perspective, and therefore, it is difficult to determine that the sphere is approaching the observer. This is because, even when the distance between the observer and the sphere changes, the size of the sphere as seen by the observer does not largely change.
In a wide observation field angle as shown in FIGS. 13A to 13C, the change in the size of the sphere as seen by the observer when the distance between the observer and the sphere changes is large, and therefore, perspective is easily obtained. Therefore, the observer can get a sense of perspective in the observation field of view. As a result, it is possible to obtain a sense of depth in the images displayed on a two-dimensional monitor that is not capable of displaying three-dimensionally because the contribution made by this perspective is substantial.
Since surgical microscopes generally obtain depth information about the object under examination based on binocular parallax, it is difficult to obtain perspective for an object moving within the observation field of view. Therefore, while perceiving an extremely unnatural sense of depth that is different from the sense of depth perceived when observing an object with the naked eye, such an effect must be continuously corrected for in the observer's brain while carrying out the surgical procedure, which causes the observer to become extremely fatigued.
Of course, a similar problem also exists even in three-dimensional medical imaging apparatuses in which the eyepiece optical system in the surgical microscope is replaced with an imaging device and in three-dimensional medical observation apparatuses for displaying an acquired image on a three-dimensional display device and observing it.